Adolescent substance abuse results in significant negative outcomes and extraordinary costs for youths, their families, communities, and society. Moreover, rates of psychiatric comorbidity among substance abusing youth are high and youths with a dual diagnosis are more costly to treat. To date, however, no outpatient treatments have been tested specifically for treating youth with co-occurring substance use disorders and mental health diagnoses. The current project aims to adapt and evaluate Multisystemic Therapy (MST), a well validated treatment for chronic behavioral problems or serious emotional disturbance in adolescents, to treat dually diagnosed youth using an outpatient model of service delivery. In Phase I of this research project, the standard MST treatment manual, supervision manual, quality assurance protocol, and therapist training protocol will be adapted for use in outpatient settings to treat dually diagnosed youth. In Phase II, the revised protocols will be included in a pilot trial with 40 dually diagnosed youth randomly assigned to treatment conditions, with 20 receiving Outpatient MST and 20 receiving usual outpatient services. Comprehensive multimethod, multisource assessments will be conducted pretreatment, 6 months post entry, and 12 months post entry to evaluate treatment efficacy. Specific aims are: Specific Aim 1: The primary aim of the present research is to adapt and test MST for use in outpatient settings to treat youth diagnosed with a substance use disorder and comorbid internalizing disorder. We hypothesize that youth receiving Outpatient MST will exhibit significantly less drug use (e.g., youth self-reports and urine screens) at 6- and 12-month follow-up than control youth who receive usual services, and that youth receiving Outpatient MST will exhibit significant improvement on indices of mental health (e.g., combined youth and caregiver reports on diagnostic interviews and youth, caregiver, and teacher reports of internalizing symptoms) at 6- and 12-month follow-up compared to control youth Specific Aim 2: In addition to improved symptomatology, the current research aims to test the effectiveness of Outpatient MST to improve youth functioning in other domains pertinent to successful adolescent development. We hypothesize that youth receiving Outpatient MST will evidence improved behavioral (e.g., youth, caregiver, and teacher reports of externalizing), school (e.g., school attendance), and family functioning (e.g., youth and caregiver reports of family adaptability and cohesion) at 6- and 12-month follow-up compared to control youth who receive usual services. Specific Aim 3: The final aim of the proposed research is to provide services that are more acceptable to consumers than are usual services provided in the community. We hypothesize that youth and families receiving Outpatient MST will experience significantly greater consumer satisfaction than control youth and families who receive usual services.